The invention concerns a camera closeup focusing mechanism for objectives of long focal length, comprising a tube between the objective housing and a mounting means for the camera, as well as anti-rotation means.
Telephoto objectives, objectives of long focal length, as a rule have focusing ranges of infinity to about 7 meters. However it is often desirable, when photographing birds and similar subjects, to be able to focus sharply at a distance of from two to three meters. This is achieved by inserting a closeup focusing mechanism between the objective housing and the mounting means for the camera.
The design of a closeup focusing mechanism in the form of a tube is already known. There is a certain minimum inner diameter which the tube must have, and should exceed, depending on the format and dimensions of the objective, in order to avoid underexposure of the border regions of the photograph.
At the same time the outer diameter of the tube imposes a limit on the dimensions of the objective and/or its fine-focusing mechanism. If the outer diameter of the tube is increased, all the dimensions of the objective must be modified. It is clear that the outer diameter of the tube is limited by these conditions
Known closeup focusing mechanisms use a tube which has the maximum possible thickness taking into account the above conditions. Grooves are machined into this tube in order to supply the required guiding means between the camera on the one side and the telephoto objective on the other.
Such guides or guide grooves are necessary in order to have means of transmitting rotational forces between the camera and the objective. This is a factor, for example, when the camera is being turned between vertical and horizontal formats, or in the attaching of the objective to the camera.
Cameras currently used for long-focal-length objectives, in particular, have substantial size and weight, typically with, for example, motorized film transports, etc. Also the weight of the telephoto objective itself is relatively high. The tube thus must form a connection between relatively heavy parts.
Since however the tube wall should be kept as thin as possible, the guide grooves in the tube must be kept small. The guides, grooves and the like are therefore subject to relatively high stresses. If the transmission and accommodation of these stresses is unsatisfactory, the guides can be overloaded, and there is danger of sudden or rapid wear. The useful life of the tube with guides therefore tends to be less than that of other parts of the camera or the telephoto objective.
An object of the present invention is to provide a closeup focusing mechanism for objectives with long focal lengths, which mechanism has a relatively high load-carrying capability and does not develop appreciable evidence of wear after long use. A further object of the invention is to provide a device which does not require an increase in the dimensions of the telephoto objective, particularly its diameter; but rather, such a closeup focusing mechanism should be usable with available telephoto objectives.